The present invention relates to a reusable printing form, in particular for use in offset printing, including a printing area, and to a method for imaging a reusable printing form.
Printing forms are used in printing units of printing presses to apply a predetermined printing pattern, a predetermined subject or image, to a printing substrate. Typical printing substrates are paper, paperboard, cardboard, organic polymers, textiles, or the like. In this context, the printing forms predominantly used are those on whose printing area, i.e. a part of the printing form surface, the pattern to be printed is permanently applied, patterned or written. Printing forms of this kind can only be used, i.e. imaged or written, once. For different reasons, it is desirable to use printing forms that can be used repeatedly, in particular, written repeatedly or imaged repeatedly. In other words, of particular interest are printing areas that can be erased after patterning into a first image and later patterned into a second image. In the context of this description, a “reusable printing form” is understood to be a printing form having a printing area that can be repeatedly patterned into different images.
In offset printing, the printing area is patterned into regions having different wetting properties, in particular, hydrophilic/lipophobic and hydrophobic/lipophilic regions. Offset printing is based on making use of the immiscibility of lipophilic substances, in particular of oily fluids or liquids, and hydrophilic substances, in particular of aqueous fluids or liquids, on the printing form, the lipophilic substance or the ink or printing ink being retained by the image-forming regions and the hydrophilic substance or water being retained by the non-image forming regions of the printing area. When wetting the suitably prepared printing area with hydrophilic and lipophilic substances, then the non-image regions preferably retain the hydrophilic substance and repel the lipophilic substance while the image regions take up the lipophilic substance and repel the hydrophilic substances. Subsequently, the lipophilic substance is then transferred in a suitable manner onto the surface of a material on which to fix the image. In waterless offset printing, the printing area is also patterned into regions having different wetting properties in a corresponding manner.
In the literature, different concepts for reusable, in particular, rewritable printing forms are presented and discussed.
In European Patent Application No. EP 0 911 154 A1, the materials proposed for a surface of a printing form are titanate (TiO2) or zirconate (ZnO2), which can be present in ceramic form, both pure or mixed with other metallic additives in different ratios. In the non-excited state, this surface is hydrophobic and capable of being transformed into a hydrophilic state by irradiation with ultraviolet light. This switching process can be reversed by heating. The imaging is now accomplished in that the entire surface of the plate is illuminated with ultraviolet light and regions which are intended to carry ink during printing are covered with a mask or a film. For erasure, the image regions are subsequently switched back, for example, using a laser beam.
The hydrophobicity of such a metal oxide surface is based in particular on a hydrocarbon-contaminated surface in air, as can be established by measurements using Fourier transform infrared spectroscopy (FTIR), X-ray photoelectron spectroscopy (XPS), or using atomic force microscopy (AFM), or the like. The surface can, in fact, be hydrophilized using UV radiation or wet chemistry, but will be hydrophobized again in an uncontrolled manner within a few hours when stored in air. Consequently, there is no defined, permanent hydrophobicity as a starting state.
Moreover, it is known, for example, from European Patent Application No. EP 0 962 333 A1 to use printing forms whose printing pattern is changeable. In this context, hydrophobic or hydrophilic materials are applied to the printing form surface, whereupon the printing form surface is wetted with water, and then ink is applied to the printing form surface. Due to the hydrophilic or hydrophobic properties, water is attracted in the hydrophilic surface regions during the wetting process with water so that the hydrophilic surface regions will not take up any more printing ink during the subsequent coating with printing ink. After a predetermined number of press runs, the applied printing pattern is removed. After that, a new printing pattern can be patterned or written on the printing form. In this context, it is known to use a thiol compound as the material for the coating of the printing form surface. The thiol compound is removed from the printing form surface under the action of heat.
When proceeding according to the technical teaching of European Patent Application No. EP 0 962 333 A1, the production of a defined and highly ordered monomolecular layer on arbitrary untreated surfaces is very complex and time-intensive. In particular, the cost of the gold substrate and of the used self-organizing molecules with thiol groups (—SH) are an obstacle to a possible technical application.